You need to edit ERA/[RMC-RMFRP]/Configuration/Herbs.XML
There are examples in the file provided, but just in case, the format is:
<Herb
name="NAME"
codes="USE SHADOW WORLD 4TH EDITION FORMAT"
form="ANY TEXT"
preparation="ANY TEXT"
cost="ANY TEXT"
description="ANY TEXT"
addictionFactor="ANY TEXT"
/>
Let me know if you need any additional detail or help about this.
And thanks for asking!